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Title: GR8677.022
Post by: conantee on December 31, 2009, 12:00:16 PM
22. Which of the following statements most accurately describes how an electromagnetic field behaves under a Lorentz transformation?
(A) The electric field transforms completely into a magnetic field.
(B) If intially there is only an electric field, after the transformation there may be both an electric and a magnetic field.
(C) The electric field is unaltered.
(D) The magnetic field is unaltered.
(E) It cannot be determined unless a gauge transformation is also specified.

Title: Re: GR8677.022
Post by: conantee on December 31, 2009, 12:09:25 PM
In order to do this question, you just need to know what Lorentz transformation is about. Just about, not even equations etc.
Well, we know from relativity that Lorentz transformation tells how physical quantities change when the observer moves with constant speed.

In case of electric and magnetic field, does it change when the observer moves?

Imagine two lines of uniformly distributed charge staying at rest. Each line will experience the force from the electric field. However, as we move along the charge in a new reference frame, here B field appears since the line of charges become currents in this frame. Now, the magnetic field appears and cause another force. However, according to principle of relativity, in both frames, two lines should experience same physical reality, i.e., experiences the same amount of force. Hence, the E field should also change in a new moving frame. In other words, the electric field can be transformed into E and B field.

Hence, the answer is (B). You don't need to case what a gauge transformation is.

The correct answer is (B), and by stats, 58 of 100 got the right answer.